FOPEN(3) Linux Programmer’s Manual FOPEN(3)
NAME
fopen, fdopen, freopen - stream open functions
SYNOPSIS
#include <stdio.h>
FILE *fopen(const char *path, const char *mode);
FILE *fdopen(int fildes, const char *mode);
FILE *freopen(const char *path, const char *mode, FILE *stream);
DESCRIPTION
The fopen() function opens the file whose name is the string pointed to by path and associates a
stream with it.
The argument mode points to a string beginning with one of the following sequences (Additional
characters may follow these sequences.):
r Open text file for reading. The stream is positioned at the beginning of the file.
r+ Open for reading and writing. The stream is positioned at the beginning of the file.
w Truncate file to zero length or create text file for writing. The stream is positioned at
the beginning of the file.
w+ Open for reading and writing. The file is created if it does not exist, otherwise it is
truncated. The stream is positioned at the beginning of the file.
a Open for appending (writing at end of file). The file is created if it does not exist.
The stream is positioned at the end of the file.
a+ Open for reading and appending (writing at end of file). The file is created if it does
not exist. The initial file position for reading is at the beginning of the file, but out-
put is always appended to the end of the file.
The mode string can also include the letter ‘‘b’’ either as a last character or as a character
between the characters in any of the two-character strings described above. This is strictly for
compatibility with C89 and has no effect; the ‘‘b’’ is ignored on all POSIX conforming systems,
including Linux. (Other systems may treat text files and binary files differently, and adding the
‘‘b’’ may be a good idea if you do I/O to a binary file and expect that your program may be ported
to non-Unix environments.)
Any created files will have mode S_IRUSR|S_IWUSR|S_IRGRP|S_IWGRP|S_IROTH|S_IWOTH (0666), as modi-
fied by the process’ umask value (see umask(2)).
FREAD(3) Linux Programmer’s Manual FREAD(3)
NAME
fread, fwrite - binary stream input/output
SYNOPSIS
#include <stdio.h>
size_t fread(void *ptr, size_t size, size_t nmemb, FILE *stream);
size_t fwrite(const void *ptr, size_t size, size_t nmemb, FILE *stream);
DESCRIPTION
The function fread() reads nmemb elements of data, each size bytes long, from the stream pointed
to by stream, storing them at the location given by ptr.
The function fwrite() writes nmemb elements of data, each size bytes long, to the stream pointed
to by stream, obtaining them from the location given by ptr.
For non-locking counterparts, see unlocked_stdio(3).
RETURN VALUE
fread() and fwrite() return the number of items successfully read or written (i.e., not the number
of characters). If an error occurs, or the end-of-file is reached, the return value is a short
item count (or zero).
fread() does not distinguish between end-of-file and error, and callers must use feof(3) and fer-
ror(3) to determine which occurred.